1. Field of the Invention
The invention relates to apparatus for electroplating selected portions of a web, and more particularly, to registration sensor means for sensing when an intermittently moving web is in registration with a plating mask in a plating head.
2. Description of Related Art
Certain articles of manufacture are provided in a strip web format for electroplating selected portions thereof. For example, lead frames used in the manufacture of integrated circuits are typically stamped from a sheet of conductive metal such a nickel, copper or the like, in the shape of a strip. Each lead frame has a separate group of leads for each chip, and the outer ends of each lead are interconnected to carrier strips which form the two edges of the web. The leads are also interconnected intermediate their ends by relatively narrow support strips. The carrier strips are perforated for feeding them through various treating operations. For completion of the integrated circuit, the inner end of each of the leads is bonded to a respective pad on the chip, and both the carrier and the support strips are severed to separate the individual leads.
In prior art techniques, a layer of precious metal, such as gold, has often been formed by electroplating one surface of the leads in order to improve bondability with the wires that connect them to the integrated circuit chips. Further, in order to minimize the amount of precious metal used, the precious metal has been plated selectively only at the portions of the leads nearest the chips.
One way of plating only the selected portions of a strip of metal, is to guide the strip through a plating head along a path of travel. The strip is intermittently advanced a certain distance, and when it stops, the plating head clamps the strip so that plating electrolyte may be washed against the surface of the strip from apertures in a plating mask in the plating head. The mask apertures are of precise size, shape and location relative to each other so that electrolyte is channeled only to the portions of the strip that are desired. However, if the strip is to be plated in the exact right spot, it is necessary that the strip always be positioned correctly relative to the mask when the strip comes to rest. To accomplish this, a mechanism is used to ascertain the longitudinal position of the strip relative to the plating head along the path of travel, and additional mechanisms move either the strip or the head so as to achieve the correct position.
In Laverty U.S. Pat. No. 4,409,924, hereby incorporated by reference, a plating apparatus is described which uses mechanical means for ensuring proper registration. In Laverty, a moveable pawl is mounted on the plating mask and is moved into engagement with indexing holes in the metal strip just as the strip is completing its movement. This causes the entire mask to float with the strip during the last small increment of strip movement and come to rest in the correct alignment with the strip. The mask is constructed as a separate lightweight structure that can slide a short distance relative to the overall plating head.
However, mechanical registration means which employ a pawl or pin to find a hole in the web, such as the pawl technique described in Laverty, are disadvantageous because the pawl or pin can scratch the metal of the web as the web moves over it. The pin can also bend the metal of the web undesirably. Further, such mechanisms can be costly and require service for moving parts.
In Corby U.S. Pat. No. 3,957,614, hereby incorporated by reference, a selective plating apparatus for lead frames is disclosed which includes an electro-optical sensing device 82. The sensing device includes a light source and a photocell facing each other from opposite sides of the web, and disposed along the path of travel such that certain apertures, spaced at predefined positions along a carrier strip of the web, periodically expose the photocell to light emitted by the light source as the web moves. The electro-optic sensor is mounted separately from the plating head as shown in Corby's FIG. 2, possibly to avoid corrosion by the caustic chemicals used in the plating process. Corrosion is less of a problem in the mechanical sensing devices, since the pawl or pins used therein may be made of corrosion resistant materials.
The sensing means disclosed by Corby does not risk scratching or bending the metal of the web, but instead has other disadvantages. In particular, since the sensor is mounted separately from the plating head, there is no rigid connection between them. Flexing of various of the supports can cause relative movement longitudinally along the path of travel of the web between the sensor and the particular locations of the web which will be plated by the plating head. Any such relative movement will cause an inaccuracy in the positions on the metal web which become plated. Nor does Corby in any way protect the sensor from caustic chemicals.
Other patents which disclose various registration means for a selective plating head include Husain U.S. Pat. No. 4,392,935; Johnson U.S. Pat. No. 3,723,283; Husain U.S. Pat. No. 4,505,225; Graham U.S. Pat. No. 3,860,499; Jogwick U.S. Pat. No. 3,974,056; and Japanese Patent Publication No. 55079894, all of which are incorporated by reference herein.
Accordingly, there is a need in the selective plating industry for a method of sensing the longitudinal position of a subject web, accurately and without risk of damaging the web.